Lab D: Thermoelectric energy conversion

Thermoelectric energy conversion
Objective
The objective of this laboratory is for you to explore the physics and practical aspects of solid‐sate heat
pumps, the direct conversion of thermal‐to‐electrical energy using thermoelectric materials, and the
thermodynamic limits to the efficiency of heat engines.
Preparation
Read the article, “Simple experiments with a thermoelectric module,’’ that is posted at the Compass
web‐site.
Equipment and samples
• Peltier device with integrated air heat exchanger
• Two dc power supplies; power supply for running the cooling fan
• Thermocouples, heaters, load resistors or decade box, hook‐up wire, breadboard
• Hand‐held multi‐meter
• Computer, data acquisition board, plotting software
Introduction
Most large‐scale conversion of thermal‐to‐electrical energy uses a gas cycle of some type. For example,
heat generated by burning coal is used to vaporize water that drives a turbine and generator. And the
air conditioner in your apartment uses electrical energy to run a compressor to condense a refrigerant.
The electrons and holes in a semiconductor can also be used to convert heat to electrical power or the
electrons and holes can be used to move heat against a temperature gradient. Semiconductors that do
this relatively efficiently are referred to as “good thermoelectrics”. The best thermoelectric material for
use near room temperature is Bi 2 Te 3 and related alloys; for many decades, researchers have been
looking for something better without success. In this lab, you will measure the efficiency of a Bi 2 Te 3 ‐
based thermoelectric module and compare this efficiency of the module to the theoretical limit, i.e., the
efficiency of the Carnot cycle.
Session 1: Peltier current and solid­state heat pumps
• Explore heat Peltier cooling and heating by applying current to the thermoelectric module in
both directions, i.e., measure the steady‐state temperature drop across the module as a
function of electrical current. Keep in mind that the Peltier heat current is linear in the
electrical current but Joule heating due to the finite resistance of the thermoelectric
materials is quadratic in the current. You will want to use the fan‐driven heat exchanger to

keep one side of the module near room temperature. Make sure that you find the minimum
cold‐side temperature so that you can estimate the effective value of the thermoelectric
figure‐of‐merit for this module from the maximum cooling.
• Measure the Peltier coefficient of the module by balancing the Peltier current against the
heat dissipated by the planar heater that is attached to the module. In other words, apply a
current to the module so that the heated side of the module stays at room temperature.
Think about what values of the heater power—and therefore what values of the electrical
current applied to the module—are needed for this measurement to be accurate and mostly
free of the effects of Joule heating within the module. Another option is to measure the
current for a large range of heater powers and use a fit to the data to determine the linear
term that gives the Peltier coefficient. Take note of the resistance of the module; you will
need that resistance in the second session.
Session 2: Seebeck coefficient and thermoelectric power generation
• Measure the Seebeck coefficient of the module by using the heater to create a temperature
drop across the module. You will want to measure the voltage as a function of the
temperature drop; the slope of that curve is the Seebeck coefficient. Is the Seebeck
coefficient constant, i.e., independent of the size of the temperature drop? Does the
Seebeck coefficient agree with the Peltier coefficient? Recall that Π = ST .
• Use the thermoelectric module to generate power from the heat dissipated by the planar
heater. Leave the fan off for this part of the experiment. You will need to pick an
appropriate load resistance to maximize the power generation. (Recall that you measured
the resistance of the module in the first session.) Measure the power generated by the
thermoelectric module as a function of heater power and compare the result to the Carnot
efficiency. (To make that comparison, you will need to record the hot‐side and cold‐side
temperatures of the module.) If you have time, repeat the experiment with the fan turned
on and measure the power required to operate the fan. How large of a temperature drop is
needed just to run the fan?
October 17, 2009